The desirability of protecting electrical meters and related equipment from transient voltage surges is well known. Earlier efforts to do so relied mainly upon current-limiting devices, such as fuses and circuit breakers, which open the circuit at excessive current flows. Examples of such use in a switch housing adjunct to a watt-hour meter appear in St. John U.S. Pat. No. 2,606,232.
Voltage-limiting protection can be provided by devices whose electrical resistance varies non-linearly under applied voltage so that conduction therethrough is slight at normal power voltages but disproportionately high at higher voltages. "Varistor" is commonly--and suggestively--applied as a name for such devices.
Varistors connected to provide such surge protection, as by shunting excess surge voltage and resulting current to an external ground, have been installed within meters, as in Melanson U.S. Pat. No. 3,914,657 and in Zisa U.S. Pat. No. 3,725,745; also in plug-and-jack devices adapted for use between electrical outlets and appliances to be operated from such outlets, as in Orfano U.S. Pat. No. 4,089,032.
The present inventor was the first person to provide voltage-limited surge-protection apparatus of the visitor type in meter adapters, as disclosed in his copending U.S. patent application Ser. No. 923,524 filed 28 Oct. 1986--which is incorporated herein by this reference. He has also devised a number of varistor arrangements for protecting watt-hour meters and metered downstream electrical loads from the deleterious effects of such surges. The electrical utility industry, thus awakened to the practicality of doing so, is now taking such protective steps increasingly rapidly.
Whatever degree of protection varistors may provide against electrical surge damage, the prior art has made little provision to save varistors themselves when subjected to surge overloads, which in severe cases can overheat any varistor to the point of failure. The present inventor's patent application mentioned above discloses heat-sink means to provide a degree of protection as surge currents impart thermal energy to the varistors and their local environment. However, a substantial need remains for augmenting protection of varistors or similar over-voltage surge-protection devices, and the present invention is directed to meeting that need in a noval way.